Encoders provide a measurement of the position of a component in a system relative to some predetermined reference point. Encoders are typically used to provide a closed-loop feedback system to a motor or other actuator. For example, a shaft encoder outputs a digital signal that indicates the position of the rotating shaft relative to some known reference position that is not moving. A linear encoder measures the distance between the present position of a moveable carriage and a reference position that is fixed with respect to the moveable carriage as the moveable carriage moves along a predetermined path.
An absolute shaft encoder typically utilizes a plurality of tracks on a disk that is connected to the shaft. Each track consists of a series of dark and light stripes that are viewed by a detector that outputs a value of digital 1 or 0, depending on whether the area viewed by the detector is light or dark. An N-bit binary encoder typically utilizes N such tracks, one per bit. An incremental encoder typically utilizes a single track that is viewed by a detector that determines the direction and the number of stripes that pass by the detector. The position is determined by incrementing and decrementing a counter as each stripe passes the detector.
In both types of encoder, the ultimate resolution is determined by the stripe pattern and the size of the detectors used to view the stripe pattern. To provide increased resolution, the density of the stripes must be increased. For example, in a shaft encoder, the number of stripes per degree of rotation must be increased. Similarly, in a linear encoder, the number of stripes between the limits of the linear motion must be increased. However, there is a practical limit to the density of stripes that is set by optical and cost constraints.
One method for providing increased resolution is to utilize an interpolation scheme to provide an estimate of the relative position of the code strip and the photodetectors between the points at which the edges of the stripes are detected. In one scheme, the light from a track is viewed by two sets of detectors. The shape and position of the detectors is set to provide two sinusoidal signals that are 90 degrees out of phase. By measuring the points at which two signals based on these sinusoids cross, intermediate position measurements can be provided. The accuracy of the determination of the intermediate points depends on the frequency of the sinusoids, which, in turn, is set by the number of stripes in the code strip or disk. Hence, this type of system can only extend the resolution by a limited amount.